Composite electrode for a plasma arc torch

ABSTRACT

A plasma arc torch that includes a torch body having a nozzle mounted relative to a composite electrode in the body to define a plasma chamber. The torch body includes a plasma flow path for directing a plasma gas to the plasma chamber in which a plasma arc is formed. The nozzle includes a hollow, body portion and a substantially solid, head portion defining an exit orifice. The composite electrode can be made of a metallic material (e.g., silver) with high thermal conductivity in the forward portion electrode body adjacent the emitting surface, and the aft portion of the electrode body is made of a second low cost, metallic material with good thermal and electrical conductivity. This composite electrode configuration produces an electrode with reduced electrode wear or pitting comparable to a silver electrode, for a price comparable to that of a copper electrode.

RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Application No.60/274,837, filed Mar. 9, 2001. The entire disclosure of thisapplication is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a composite electrode for a plasma arctorch. In particular, the invention relates to a composite electrode fora plasma arc torch in which a forward portion of the electrode bodycomprises a first metallic material having high thermal conductivity andthe remaining aft portion of the electrode body comprises a second lowcost, metallic material with good thermal and electrical conductivity.

BACKGROUND OF THE INVENTION

Plasma arc torches are widely used in the cutting or marking of metallicmaterials. A plasma torch generally includes an electrode mountedtherein, a nozzle with a central exit orifice mounted within a torchbody, electrical connections, passages for cooling and arc controlfluids, a swirl ring to control fluid flow patterns in the plasmachamber formed between the electrode and nozzle, and a power supply. Thetorch produces a plasma arc, which is a constricted ionized jet of aplasma gas with high temperature and high momentum. Gases used in thetorch can be non-reactive (e.g. argon or nitrogen), or reactive (e.g.oxygen or air).

In operation, a pilot arc is first generated between the electrode(cathode) and the nozzle (anode). Generation of the pilot arc may be bymeans of a high frequency, high voltage signal coupled to a DC powersupply and the torch or any of a variety of contact starting methods.

One known configuration of an electrode for a plasma arc torch includesan emitting insert (e.g., hafnium) which is press fit into a bore in theelectrode. An objective in electrode design is to transfer heat from thehafnium insert and into a cooling medium, which is usually water.Another objective is to control arc root attachment to minimize erosioncaused by undesirable arc root attachment to the electrode instead ofthe hafnium insert.

Electrodes for plasma arc torches are commonly made from copper. Copperis a low cost material that offers good thermal and electricalconductivity. Electrodes for plasma arc torches can also be made fromsilver. While silver electrodes provide excellent heat transfercharacteristics, they tend to be very expensive and not cost effectiveto use. Copper electrodes are cost effective, but do not have thesuperior heat transfer characteristics of a silver electrode and thushave a shorter electrode life than silver electrodes.

Several companies manufacture silver and silver/copper compositeelectrodes using a variety of manufacturing techniques includingbrazing, soldering, swaging, press fitting and other methods. Onecompany has developed a vacuum brazed copper/silver composite designwith a through-hole hafnium insert. Another company has developed apress-fitted silver annulus design with a blind hole hafnium insert.Another company has developed a swaged silver annulus design in a copperholder with copper on the front portion. Another company has developedcoined silver electrode design. However, these methods of manufacturingsilver/copper electrodes do not produce a sufficiently high-strengthjoint at the silver/copper interface. In addition, these manufacturingmethods result in electrodes that can leak cooling fluid at thesilver/copper interface. More significantly, these silver/compositeelectrodes do not offer the heat transfer characteristics of an allsilver electrode.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improvedcomposite electrode, which combines the material property benefits ofsilver with the cost benefits of copper.

Another object of the present invention is to provide an improvedcomposite electrode that does not leak cooling fluid.

In one aspect, the invention features a plasma arc torch for cutting ormarking a metallic workpiece. The torch includes a torch body having anozzle mounted relative to a composite electrode in the body to define aplasma chamber. The torch body includes a plasma flow path for directinga plasma gas to the plasma chamber. In one embodiment, the torch canalso include a shield attached to the torch body. The nozzle, compositeelectrode and shield are consumable parts that wear out and requireperiodic replacement.

The composite electrode has two portions made from different materials.The forward portion of the electrode comprises a metallic material withexcellent heat transfer properties (e.g., high thermal conductivity)(e.g., silver). An emissive insert (e.g., hafnium, zirconium, tungsten,thorium, lanthanum, strontium, or alloys thereof) is disposed in a borein the forward portion. The aft portion of the electrode comprises a lowcost, metallic material with good heat transfer properties (e.g., goodthermal conductivity) (e.g., copper).

The high thermal conductivity, forward portion is joined onto an end ofthe good thermal conductivity, aft portion to form the compositeelectrode. The two portions are joined by a direct welding process, suchas friction welding, inertia friction welding, direct drive frictionwelding, CD percussive welding, percussive welding, ultrasonic welding,or explosion welding, that forms a hermetic seal between the twoportions of the electrode. To maximize cooling, the forward portion alsoextends back to the area of cooling fluid flow and is therefore directlycooled by the fluid. This construction, in contrast to known electrodedesigns having a relatively small diameter, high thermal conductivitysleeve inserted into a cavity formed in the front end for surrounding anemissive insert, is believed to provide an electrode that has superiorheat transfer properties and does not leak cooling fluid.

In another aspect, the invention features a composite electrode for aplasma arc torch for cutting or marking a metallic workpiece. Thecomposite electrode includes a forward portion comprising a metallicmaterial with excellent heat transfer material properties (e.g., highthermal conductivity) (e.g., silver). The aft portion of the electrodecomprises a low cost, metallic material with good heat transfer materialproperties (e.g., good thermal conductivity) (e.g., copper).

The high thermal conductivity, forward portion is joined onto an end ofthe good thermal conductivity, aft portion to form the compositeelectrode. In one embodiment, the forward and aft portions are in directcontact at the mating surface. To accomplish this, the two portions arejoined together by a direct welding technique—such as friction welding,inertia friction welding, direct drive friction welding, CD percussivewelding, percussive welding, ultrasonic welding, or explosion welding.The direct welding process forms a high strength, hermetic seal betweenthe two portions of the electrode. To maximize cooling, the forwardportion also extends back to the area of cooling fluid flow and istherefore directly cooled by the fluid.

Yet another aspect of the invention features a method of manufacturingan electrode for cutting or marking a workpiece. An electrode isprovided including a forward portion comprising a metallic material withexcellent heat transfer material properties (e.g., high thermalconductivity) (e.g., silver). An aft portion of the electrode body isalso provided, comprising a low cost, metallic material with good heattransfer material properties (e.g., good thermal conductivity) (e.g.,copper). The two portions of the electrode are joined by a directwelding technique. They can be joined, for example, by friction welding,inertia friction welding, direct drive friction welding, CD percussivewelding, percussive welding, ultrasonic welding, or explosion welding,thereby forming a high strength, hermetic seal between the forward andaft portions of the electrode. Cooling fluid flow can be used to coolthe forward portion of the electrode, and an insert with high thermionicemissivity can be located in a bore in the forward portion of theelectrode body.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of one embodiment of a plasma arc torchaccording to the invention.

FIG. 1A is a cross-sectional view of one embodiment of a compositeelectrode for use in the plasma arc torch of FIG. 1.

FIG. 2 is a cross-sectional view of another embodiment of a compositeelectrode for use in the plasma arc torch of FIG. 1.

FIG. 3 is a graph comparing the number of starts vs. pit wear fromvarious electrode configurations.

FIG. 4 is a graph comparing the performance of electrodes according tothe present invention with other electrodes, using 4 second testing.

FIG. 5 is a graph comparing the performance of electrodes according tothe present invention with other electrodes, using 60 second testing.

FIG. 6 is a graphical representation showing a temperature contour plotin a model of a silver tip electrode during torch operation, based on acomputational fluid dynamics model.

FIGS. 7A-7Q show various embodiments of electrode tip configurations ofthe invention.

FIG. 7R shows an aft portion of an electrode having a receiving portion.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a plasma arc torch 10 embodying the principles of theinvention. The torch has a body 12, which is typically cylindrical withan exit orifice 14 at a lower end 16. A plasma arc 18, i.e. an ionizedgas jet, passes through the exit orifice and attaches to a workpiece 19being cut. The torch is designed to pierce, cut, or mark metal,particularly mild steel, or other materials in a transferred arc mode.In cutting mild steel, the torch operates with a reactive gas, such asoxygen or air, as the plasma gas to form the transferred plasma arc 18.

The torch body 12 supports a composite electrode 20 having a generallycylindrical body 21. A hafnium insert 22 is disposed in the lower end 21a of the composite electrode 20 so that a planar emission surface 22 ais exposed. The insert 22 can also be made of other materials possessingsuitable physical properties, such as corrosion resistance and a highthermionic emissivity. In one embodiment, the insert material has anelectron work function of about 5.5 electron volts or less. Suitablematerials include hafnium, zirconium, tungsten, yttrium, iridium, andalloys thereof. The torch body also supports a nozzle 24, which isspaced from the composite electrode. The space between the nozzle 24 andthe composite electrode 20 defines a plasma chamber 30. The nozzle 24has a central orifice that defines the exit orifice 14. A swirl ring 26mounted to the torch body has a set of radially offset (or canted) gasdistribution holes 26 a that impart a tangential velocity component tothe plasma gas flow causing it to swirl. This swirl creates a vortexthat constricts the arc and stabilizes the position of the arc on theinsert.

There are two ways to start the torch. One solution has been contactstarting, one form of which is described in U.S. Pat. No. 4,791,268.However, a principal starting technique currently in use uses a highfrequency, high voltage (HFHV) signal coupled to a power line from aD.C. power supply to the torch. The HFHV signal induces a sparkdischarge in a plasma gas flowing between the composite electrode and anozzle, typically in a spiral path. A HFHV generator is usuallyincorporated in a power supply or in a “console” located remotely fromthe torch and connected to the torch by a lead set.

The arc between the electrode and nozzle is a pilot arc, and the arcbetween the composite electrode and the workpiece is a transferred arc.The gas flow through the nozzle is ionized by the pilot arc so that theelectrical resistance between the composite electrode and the workpiecebecomes very small. Using a pilot resistor, a higher voltage is appliedacross the composite electrode and the workpiece to induce the arc totransfer to the workpiece after the gap is ionized. The time betweenstarting the pilot arc and transferring to the work is a function of thedistance of the torch above the work, the pilot arc current level, andthe gas flow rate when the traditional start circuits are used.

Electrodes have been commonly manufactured from copper. Copper has beenchosen because of its good heat transfer capabilities and low cost.Applicants have determined that significant improvements in the servicelife of electrodes can be achieved using a high purity all-silver orcoined silver electrode (e.g., 90% silver, 10% copper) with a swagedhafnium emitting element. Test results have shown over 2000 starts forsuch an electrode in laboratory testing with a plasma arc torchoperating using a non-ramp-down process. This type of electrode allowsdirect water cooling of the silver surrounding the hafnium. However, dueto the high material cost of silver, this electrode design is veryexpensive and has not achieved wide market acceptance.

Applicants have achieved results comparable to an all-silver electrodeusing a copper/silver composite electrode in accordance with the presentinvention. To accomplish this, Applicants have optimized the amount ofsilver through material analysis, steady state heat flux modeling andempirical data collection. Applicants' test results show thatsignificant gains in electrode service life can be realized if thesilver component extends from the forward portion of the electrode backinto the area of the hollow mill and is directly cooled by water. In oneembodiment, both the hafnium insert 22 and the silver are directlycooled by water.

FIG. 1A shows a cross-sectional view of one embodiment of a compositeelectrode 20, in which the hafnium insert 22 can be directly cooled by acoolant 52 such as cooling water. The coolant circulates through aninternal flow path inside of the composite electrode, including interiorsurfaces of the aft portion 20B, and across interior surfaces of theforward portion 20A, including the bottom wall 42A and side walls 42B.The cooling fluid exits the composite electrode via the annular passage54 defined by the tube 58 and the inner wall 59 of the electrode 20. Thecomposite electrode is also preferably “hollowmilled.” That is, it hasan annular recess 56 formed in the interior surface of the bottom wall42A, to enhance the surface area of the body material, thereby promotinga heat exchanging relationship with the coolant 52. The planar emissionsurface 22A is sized, in conjunction with the flow of coolant 52 and thesurface areas of the bottom wall 42A and the side walls 42B and 42C, toprevent boiling of the hafnium insert 22. Further, although the insert22 is illustrated as being a single cylindrical piece, other geometrysare within the scope of the invention. Use of multiple inserts is alsocontemplated.

In its most basic form, Applicants' electrode includes a forward silverportion directly joined to an aft copper portion. A hafnium insert isdisposed in a bore formed in the forward portion. See FIG. 2, describedin detail below.

Applicants have recognized the difficulty in obtaining a high strength,leak-proof joint at the copper/silver interface when using conventionalmethods of joining, such as press-fit, soft-solder, vacuum brazing,torch brazing, threading, adhesive, ultrasonic weld, etc. Use of swaged,soft soldered, silver soldered, or induction brazed techniques used toattach the forward silver portion to the aft copper portion do notresult in a reliable hermetic seal. This occurs because the joint mustwithstand torque during assembly, high pressure coolant duringoperation, heat stress, thermal expansion and contraction, shear stress,thermal fatigue, etc.

Applicants' invention includes techniques for efficiently andeffectively joining the aft portion 20B directly with the forwardportion 20A. The aft portion 20B has a first mating surface 46 that isjoined with a second mating surface 47 of the forward portion 20A, usingtechniques such as those described below. Combination of the first andsecond mating surfaces 46 and 47 results in a joint. In one embodiment,the mating surfaces are planar, as illustrated. However, non-planarmating surfaces can be used as well. The term non-planar includes anycontour or shape that can be used, for example, with the joiningtechniques described below. In one preferred embodiment, the first orsecond mating surface has a circular, planar cross-sectional shape. Thesize of each mating surface can be the same, or they can be different.

In general, the invention contemplates a process to join directly (i.e.,without the use of any additional material) the forward and aftportions. The first mating surface 46 is joined to the second matingsurface 47, using a direct welding technique, such as friction welding,which results in the forward and aft portions being in direct contactwith each other. Friction welding is widely used to weld dissimilarmaterials and minimize cost per part. Friction welding is an idealprocess for joining dissimilar metals and provides high reliability, lowporosity, and excellent strength. Friction welding is an ideal processfor forming a high strength, leak-proof weld between silver and copper,resulting in a hermetic seal. In addition, friction welding does notrequire the use of an additional material (e.g. solder). Frictionwelding, inertia friction welding, and direct drive friction weldingtechniques, are performed, for example, by MTI Welding of South Bend,Ind., and are described on their web site. See, for example,http://www.mtiwelding.com. Pages found at this web site describe varioussuitable welding techniques, and some of the associated metalcombinations on which they can be used.

More particularly, these web pages describe friction welding techniques,including inertia friction welding and direct drive friction welding.These techniques can be used to create a joint between dissimilarmaterials that is of forged quality, and can be used to create a 100%butt joint weld throughout the contact area of the two pieces beingjoined. These and other direct welding techniques, including CDpercussive welding, percussive welding, ultrasonic welding, explosionwelding, and others, utilize combinations of workpiece acceleration anddeceleration, welding speed, frictional forces, forge forces, and othersuch physical forces, sometimes in combination with electricity atvarious voltages and current flows, to create and use force and/or heatin a predetermined and controlled manner, between the workpieces beingjoined, to create a strong, leak-proof joint without the introduction ofextraneous materials (such as flux, solder, braze, or filler materials).They accomplish this utilizing rapid and efficient cycle times, and withminimal loss of the working materials. These techniques are allconsidered to be within the scope of the invention.

Direct welding techniques, and friction welding techniques inparticular, have been successfully employed to join materials such assilver and copper, but are also effective for joining variouscombinations, for example, of the following materials, or alloysthereof: aluminum, aluminum alloys, brass, bronze, carbides cemented,cast iron, ceramic, cobalt, columbium, copper, copper nickel, ironsintered, lead, magnesium, magnesium alloys, molybdenum, monel, nickel,nickel alloys, nimonic, niobium, niobium alloys, silver, silver alloys,steel alloys, steel-carbon, steel-free machining, steel-maraging,steel-sintered, steel-stainless, steel-tool, tantalum, thorium,titanium, titanium alloys, tungsten, tungsten carbide cemented, uranium,vanadium, valve materials (automotive), and zirconium alloys. Proper useof these techniques results in the significant electrode performanceenhancements of the invention, as contrasted, for example, withconventional brazing, soldering, and other joining methods, some ofwhich were discussed earlier.

For purposes of this invention, in addition to the techniques describedabove, direct welding includes joining methods that create a suitablehigh-strength joint between the dissimilar metals of the first matingsurface 46 and the second mating surface 47, without the need to addadditional materials such as braze, flux, solder, or filler materials.For purposes of this invention, direct welding includes inertia frictionwelding, direct drive friction welding, CD Percussive welding,percussive welding, ultrasonic welding, and explosion welding. Thesemanufacturing methods achieve a direct metallurgical coupling betweenthe first and second mating surfaces, resulting in a strong bond at lowcost. The direct contact between the mating surfaces, especially in theabsence of solder, flux, braze, filler materials and the like,contributes to the superior performance of the invention. Moreover, itis recognized that an alloy may be formed where the first and secondmating surfaces meet, resulting from the combination of these differentmaterials. This alloy may be formed either during direct welding, and/orduring subsequent operation of the torch. Applicants have determinedthat formation of any alloy in this manner does not hinder theperformance of the invention. Rather, it is the use of braze, flux,solder, welding filler materials, and the like, such as those used inother types of joining processes, that should be avoided. These types ofmaterials are not used in the direct welding process of the invention,allowing Applicants to achieve the direct contact between the matingsurfaces that is required.

In one aspect, Applicants have developed an electrode with an optimalvolume and geometry of a forward silver portion and an aft copperportion based on (1) performance and (2) cost and ease of manufacturing.Applicants' composite electrode performs as if it is an all-silverelectrode. The electrode approximates the material properties of themore expensive silver material. The electrode uses the requisite volumeof silver to provide excellent heat transfer in the forward portionaround the emissive insert, to achieve performance and service lifeequal to that of the all-silver electrode. The requisite geometry andvolume can be determined through empirical data collection in thelaboratory, and by computer modeling of the heat flux. These techniquescan be used, for example, to design electrodes that minimize the amountof silver used during electrode fabrication, thereby reducing the costof the electrode. Cavities or lumens can be strategically located withinportions of the forward and/or aft portions of the electrode body, forexample, to enhance cooling capabilities, or to reduce the quantity ofmaterial required for fabrication. Applicants have also used thesetechniques to determine that superior cooling of the hafnium insert 22is achieved by providing a high thermal conductivity material, such assilver, in the forward portion 20A to surround the circumference of theemissive insert 22, thereby providing contact with the excellent heattransfer property of the forward portion of the electrode along thelength of the insert 22, whereby the life of the electrode is extended.Further, Applicants have determined that providing a single radialinterface between the insert 22 and the forward silver portion alsoresults in superior electrode performance.

The aft portion 20B of the electrode can be made with a lower costcopper material which still has good heat transfer properties, butresults in a composite electrode with performance characteristicscomparable to an all-silver electrode for a much lower cost. Inaddition, as the majority of heat transfer can take place in the forwardportion 20A, a higher emphasis on the machinability of the aft portioncan be used as a criterion in the material selection of the aft portion.The heat transfer property of the forward and aft portions of theelectrode can be, for example, thermal conductivity or thermaldiffusivity.

The forward and aft portions of the composite electrode can be made fromvarious combinations of materials. In one embodiment of the inventionthe thermal conductivity of the forward portion of the electrode (e.g.,silver) is generally greater than about 400 Watts/m/deg-K, and thethermal conductivity of the aft portion of the electrode (e.g., copper)is generally less than this amount. In another embodiment, the materialsof the forward portion of the electrode have a high thermal diffusivity,generally greater than 0.1 m²/sec., and preferably at least about 0.17m²/sec. The thermal diffusivity of the aft portion of the electrode isless than the thermal diffusivity of the forward portion. Any material,including alloys, with physical properties such as those listed abovecan be suitable for use with the invention and are contemplated to bewithin the scope of the invention.

In addition to silver/copper, other composite or multi-metalliccombinations with desirable characteristics for use with the compositeelectrode of the invention can be used. Different embodiments of theinvention can use silver/aluminum, silver/brass, or brass/coppermaterial combinations for the forward and aft portions of the electrode.Applicants usage herein of the term “composite” is intended to mean atleast two metallic materials.

FIG. 2 is an illustration of an embodiment of an electrode 200 embodyingthe principles of the present invention. The main components of theelectrode 200 are a forward silver portion 210 and an aft copper portion220, which has been friction-welded to the forward silver portion 210.The friction-welded joint is created where the surfaces of the forwardsilver portion 210 and the aft copper portion 220 meet. Although thejoint is described as friction-welded, the other direct welding joiningtechniques such as those described above can also be used, and areconsidered to be within the scope of the invention. Moreover, althoughthe forward silver portion 210 can be primarily silver, other materialssuch as gold, palladium, silver-copper alloys, brass, rhodium andplatinum, and alloys of any of these are also suitable, and are withinthe scope of the invention.

The joint illustrated in FIG. 2 has a cross-sectional area that extendsacross the width of the electrode 200. In other embodiments of theinvention, the diameters of these portions can be different, and thesecross-sectional areas can be different. Further, the shape of theforward portion 210 can be different from the shape of the aft portion220. For example, the forward portion can be in the shape of a disk or asquare, and the aft portion can be in the shape of a tube, with the endof the tube being friction-welded to a surface of the forward portion.Many various shapes and configurations are contemplated, and provide foreffective operation of the invention.

In one embodiment of the invention, the forward silver portion 210comprises or is made of silver and the aft copper portion 220 comprisesor is made of copper. The forward silver portion 210 has a bore 230 intowhich a hafnium insert can be press fit. As illustrated in FIG. 2, thebore 230 can be located along a central axis of the forward portion ofthe electrode body. The friction weld used to attach the forward silverportion 210 to the aft copper portion 220 results in a reliable,leak-proof hermetic seal along with a high strength weld. To maximizecooling, the forward portion also extends back to the area 240 ofcooling fluid flow and is therefore directly cooled by the fluid. In oneembodiment, the electrode 200 is of a hollow-milled configuration. Asshown in FIG. 2, the hollow-milled configuration results in increasedsurface area 250A, 250B, 250C, 250D, 250E, and 250F for transferringheat from a hafnium insert to cooling area 240.

Full strength welds of oxygen-free copper to coined (e.g., 90% silver,10% copper) silver have been achieved using friction welding. Bend testsand tensile tests showed strength equal to silver material. Laboratoryresults comparing pit depth of an electrode against the number ofpierces for a silver/copper electrode were identical to an all-silverelectrode, until the depth of silver was consumed, as shown in FIG. 3.The foregoing are merely representative embodiments, as otherconfigurations are possible and within the scope of the invention.

FIG. 4 is a graph that shows pit depth versus the number of electrodestarts for various electrodes. The performance of electrodes that aremanufactured according to the invention are designated as curves 401 and403 on the graph. This graph compares these results with those of copperelectrodes (405 and 406), and with other copper-silver electrodecombinations (408A-408F) that are commercially available. The data inFIG. 4 was obtained using 4 second life test testing measurements, i.e.,multiple four second runs were made with each of the electrodes, toobtain the information displayed in this graph. The graph shows thesuperior longevity of electrodes manufactured according to Applicants'invention.

FIG. 5 shows a graph of comparable data as FIG. 4, but for 60 secondlife test measurements (i.e., multiple runs of 60 seconds duration oneach of the electrodes). Electrodes according to the invention arelabeled on FIG. 5 as 501 and 503. Copper electrode results are labeledas 505 and 506. Results of commercially available copper-silvercombination electrodes are labeled as 508A-508F. Again, the resultsillustrate the superior longevity of the electrodes manufacturedaccording to Applicants' invention.

FIG. 6 is a plot showing temperature contours in a silver tip electrodeduring extended operation based on a computational fluid dynamics model.This plot presents a cross-sectional view of an operating electrodecomprising a hafnium insert 22 within a silver forward portion 20A. Theelectrode modeled in this figure is symmetrical about a central axis605. The electrode is cooled by coolant that is present in the annularrecess 56. The temperature at and near the planar emission surface 22Ais hotter than the maximum temperature reading displayed by the graph(190 deg-C), and is displayed as white (area 610). This figurequalitatively demonstrates the degree of radial heat conduction awayfrom the hafnium insert 22 in the electrode, and illustrates theimportance of having silver available in the radial direction to enhanceconduction.

Radial heat conduction away from the hafnium insert 22 is an importantfeature of the invention. FIGS. 7A-7Q illustrate some differentembodiments of different configurations of electrode tips that arewithin the scope of the invention. The diameter and/or quantity of thesilver portion of the electrode tip 705 is sized to achieve the desiredamount of radial cooling for a particular application, in combinationwith the amount and shape of the copper portion of the electrode 710,and the size, shape, and positioning of the hafnium insert 22 or inserts(if multiple inserts are present). In the embodiments of the inventionshown in FIGS. 7A-7Q the entire length of the hafnium insert 22 is incontact with the silver portion of the electrode tip 705, to facilitateheat removal.

FIG. 7R shows one embodiment where the aft portion of the electrode isadapted to receive the forward portion of the electrode. The size andshape of the aft portion of the electrode 710 can be adjusted to allowthe second mating surface 47 to fit within a receiving portion 715formed by the first mating surface 46. In this embodiment the forwardportion of the electrode tip 705 has a smaller diameter than the aftportion of the electrode tip 710, and the forward portion of theelectrode tip 705 can be fabricated to fit within the receiving portion715. The forward portion of the electrode can occupy substantially allof the diameter of the receiving portion 715. After friction welding,this embodiment of the invention can result in an electrode tip such asis depicted in FIG. 7Q.

The described embodiments preferably use coolant 52 to remove heat fromthe hafnium insert. These geometry of the forward and aft portion of theelectrode can be manipulated in combination, to optimize, for example,heat conduction requirements and manufacturing costs. The silver used inthe electrode tip is strategically located to optimize utilization ofits heat transfer property. Use of direct welding allows less expensivematerials (e.g., copper) to be used where the properties of the moreexpensive materials are not required.

While the invention has been particularly shown and described withreference to specific preferred embodiments, it should be understood bythose skilled in the art that various changes in form and detail may bemade therein without departing from the spirit and scope of theinvention.

1-50. (cancelled)
 51. A composite electrode for use in a torchcomprising: a forward portion comprising a first material; and an aftportion directly welded to the forward portion and comprising a secondmaterial different from the first material.
 52. The composite electrodeof claim 51 further comprising an emissive insert in direct contact withthe forward portion.
 53. The composite electrode of claim 52, wherein anentire length of the emissive insert is in direct contact with theforward portion.
 54. The composite electrode of claim 52, wherein theforward portion is directly cooled by a coolant.
 55. The compositeelectrode of claim 54, wherein the emissive insert is directly cooled bya coolant.
 56. The composite electrode of claim 51, wherein the firstmaterial and the second material have a heat transfer property, thevalue of the heat transfer property for the first material being greaterthan that of the second material.
 57. The composite electrode of claim51, wherein the forward portion comprises silver and the aft portioncomprises copper.
 58. The composite electrode of claim 57, wherein theforward portion comprises a silver alloy.
 59. An electrode for use in atorch comprising: a composite electrode body defining a recess, thecomposite electrode body including a forward portion comprising a firstmaterial in direct contact with an aft portion comprising a secondmaterial; and an emissive insert disposed within the composite electrodebody, wherein the insert is directly cooled by a coolant within therecess.
 60. The electrode of claim 59, wherein the first material of thecomposite electrode body has a first heat transfer property and thesecond material of the composite electrode body has a second heattransfer property, the first heat transfer property being greater thanthe second heat transfer property.
 61. The electrode of claim 59,wherein the first material comprises silver and the second materialcomprises copper.
 62. The electrode of claim 61, wherein the firstmaterial comprises a silver alloy.
 63. The electrode of claim 61,wherein the forward portion comprising the first material is directlycooled by a coolant within the recess.